Laser diodes are typically used to transmit data information over fiber optic networks. To achieve higher speed data rates, a laser diode can be biased with a drive current so it is ‘ON’ and produces at least a minimal optical output. While the diode is biased, the diode can be driven with additional current so that the light output of the diode varies over time between two power output levels. One power output level of the diode can represent a logic low or zero while another power output level of the diode can represent a logic high or one. The optical extinction ratio is the term applied to the relationship in dB between the logic one optical power level and the logic zero optical power level.
Laser diodes are typically supplied to system integrators (e.g., communication network developers) as part of a single transmitter or transceiver module that includes not only the laser diode, but also a laser driver and a controller. In the case of a transceiver, the module will also typically include a receiver photodiode and a receiver circuit that includes an amplifier. Such modules are usually supplied with standard input and output pins to connect to external circuitry. These modules were generally first designed for use with continuous mode lasers, which have been in widespread use for many years in optical communications systems that employ SONET and SDH protocols, for instance. Recently, however, burst mode lasers have come to be used in optical communication systems such as Passive Optical Networks, for example.
A burst mode laser, in contrast to a continuous mode laser, produces output only during selected intervals. It will be appreciated that the burst-mode transmitter is essentially turned off and does not transmit an optical signal until a burst-mode incoming signal is received. Only upon receiving the incoming signal will the burst-mode transmitter operate in comparison to the constant transmission of optical signals at the output of continuous mode transmitters. This manner of biasing a burst mode laser is illustrated in FIG. 1, which shows the optical power generated by the burst mode laser as a function of its bias current as well as an optical signal consisting of a series of logic ones and zeros. A first bias current is applied to the burst mode laser to achieve a logic zero level. Likewise, a second or modulation bias current is applied to the burst mode laser to achieve a logic one level. At all other times the laser is off and no bias is applied.
It will be appreciated that the incoming signals used to bias the burst mode laser can be of various lengths of data, where some signals can be as short as 10 microseconds, for instance, in the case of a DOCSIS burst signal. In a Gigabit PON (GPON) network, the minimum burst time is 32 ns including the preamble, delimiter and data. The minimum amount of data per burst is 1 byte (6.4 ns).
To compensate for temperature fluctuations and aging, many laser driver control circuits employ an analog control loop to maintain a constant average output power from the laser. A power monitor photodiode senses the output power of the laser for feedback to the driver control circuit. In particular, the power monitor photodiode typically receives a portion of the output power from the back facet of the laser and generates a current that is proportional to the output power from the front facet of the laser. The front facet of the laser is aligned with the fiber core to create a signal output path.
Similar to continuous mode laser diodes, burst mode laser diodes are also generally supplied to system integrators as part of a single transmitter or transceiver module in which additional components such as those mentioned above are integrated onto a single board. For convenience, burst mode laser diodes have often been retrofitted into modules that were originally configured for continuous mode laser diodes, with the same operational states, the same input and output pin arrangements, and the like.
It is well known that the operating characteristics of laser diodes used in each transceiver or transmitter module typically varies from module to module within a given product line. This variation prevents the utilization of global bias settings throughout an entire product line whose components and configurations are otherwise identical. Therefore, the logic zero and one bias levels for each module must be calibrated and set individually to achieve optical levels of operation. These optimal levels correspond to operations where the extinction ratio and optical output power remain at desired values. Calibrating and setting the logic zero and one bias levels of each individual laser diode for each transceiver is both expensive and time consuming.
The logic zero and one bias levels of a laser diode are normally established using a test system that contains a digital communication analyzer (“DCA”). The DCA is used to monitor an optical output signal having a random pattern of data while varying the logic zero and one bias levels of an optical transmitter module until the bias levels are found that produces the optimal extinction ratio in the optical output signal is found. At the same time the monitor photodiode is also calibrated by recording the average current generated by the monitor photodiode for both the logic zero and logic one levels corresponding to the desired extinction ratio. Accordingly, in operation, the desired extinction ratio and its corresponding optical logic zero and logic one levels can be achieved by adjusting the bias levels as necessary to maintain the current generated by the photodiode at the levels that have been recorded during the calibration process.
The DCA is essentially a specialized oscilloscope with built-in software for measuring the extinction ratio and eye mask of the laser diode. A significant disadvantage to using the DCA to set the logic zero and one bias levels is the cost of the DCA itself and the time and costs associated with the configuration of the software and parameter settings of the test system. Despite these disadvantages, DCAs are commonly used for continuous mode laser diodes, where extinction ratios in the range of about 10 dB are often required.
The use of a DCA to establish the bias levels of a burst mode laser diode and to calibrate the monitor photodiode in the laser driver circuit is particularly problematic. Burst mode lasers typically need to operate with much higher extinction ratios than continuous mode lasers. For instance, ITU specifications call for extinction ratios of 10 dB or higher and in many circumstances extinction ratios in the range of 20-30 dB are needed. Unfortunately, the measurement accuracy of a DCA is only about 15 dB for a signal with an average power greater than 0 dBm, making it unsuitable for use in establishing the bias levels of many burst mode laser diodes, particularly when its relatively high cost in both time and money is taken into account.